Alternatives to QFT

waterfall

But can you apply it to strings theory and say that a "quantum theory of gravity as spin two particles in a flat space-time leads to GR"?

In classical physics. This http://www.worldscinet.com/ijmpd/17/...1808012085.pdf shows spin 2 particles in flat spacetime CAN'T lead to GR.

How is it that in a quantum theory of gravity like String theory, spin 2 particles in flat spacetime CAN lead to GR while in classical physics, It CAN'T (as juanrga emphased in his shared paper)?

fzero

Looking at what you quote, it seems that someone else said that. Flat spacetime is just a convenient starting point. It is also possible to study strings in other backgrounds.

Actually the classical theory of a spin 2 particle (along with the appropriate linear gauge invariance) leads to equations of motion that are precisely what would have been obtained from Einstein's equation. If you study the quantum theory (as an effective theory) you will find higher-order corrections to Einstein's equations.

waterfall

But according to juanrga in his shared paper http://www.worldscinet.com/ijmpd/17/...1808012085.pdf

"There is more to gravity than gravitons. (There is sufficient evidence to assume that gravity is not a fundamental field but an emergent phenomenon like elasticity."

Please read the paper written by India top physicist which disproves that in classical GR, spin-2 fields in flat spacetime can lead to GR. If it doesn't apply classically. You can't apply it in quantum gravity classical limit.

atyy

That paper is discussing subtleties. Padmanabhan still agrees that classical GR = spin 2: "Then we need to assume that the spin 2 field ... This assumption will lead consistently to Einstein’s theory and seems to be the most viable option, if we want to obtain standard gravity coupled to matter, starting from the graviton action."

When he says gravity is more than gravitons, he is talking about quantum gravity near the Planck scale - there Padmanabhan favours emergent gravity like string theory: "There is more to gravity than gravitons. (There is sufficient evidence to assume that gravity is not a fundamental field but an emergent phenomenon like elasticity. ..."

fzero

He doesn't disprove anything of the sort. He notes that on a manifold with boundary, the linearization of the Einstein-Hilbert action includes boundary terms. This has been known for 40 years and the resolution of the problem is to add a boundary term to the EH action http://en.wikipedia.org/wiki/Gibbons..._boundary_term. The bulk+boundary action is taken as the definition of GR on a manifold with boundary and its linearization agrees with the spin 2 theory.

juanrga

Before continuing misinterpreting what I really said, please read what I wrote in #201. Thanks.

waterfall

Thanks. I understood things now more clearly than ever.

I believe with a little fixing, the quantum spin-2 field would be the primary entity and the geometry merely as a result of the symmetry in the math of the quantum field theory.

It's better than believing gravity is only geometry as General Relativity folks love to express.
Therefore I'm more inclined now toward string theory especially M-Theory which may involve what Witten describes as an incredible quantum symmetry where strings are just temporary constructs or a dual bit.

I'm not sure about Loop Quantum Gravity. If it's about geometry and reverse engineering it to get to the quantum parts or spin networks. Then it has less elegance.

waterfall

In quantum field theory, the quanta like photons and gravitons are just momentum and energy of the fields. Of course you need the entire fields to do the work although what you can measure are simply the photons and gravitons but the fields which you take as the ocean underneath the "small ripples on a water surface" need to be active with the properties it needs.

Now in the case of our spin-2 gravitons discussions. To be sure I understood the concept. Let me explain it to you. What they meant when they say spin-2 field over flat spacetime equaled curved spacetime and gravity is not that spin-2 gravitons is enough to pull off those trick. But the ocean or gravitational fields derived from a quantum gravity has the properties and right coupling to pull of the gravity act. Then the spin-2 gravitons are just manifestation of this hidden gravitational fields. Therefore to avoid confusion. I think the proper things to say is instead of:

"Spin 2 gravitons + flat spacetime = General Relativity". One must say this:

"Spin 2 gravitons (with underlying gravitational field produced from excitations of strings or LQG or others) + flat spacetime = General Relativity."

Are we clear on this now. You guys used the former descriptons all throughout hence you confused even others like "surprised". Right?

waterfall

In post #99 a week ago:


We had a long detour on string theory and spin-2 graviton thing because I was asking Marcus above (in post #98) if Loop Quantum Gravity was also about spin-2 graviton on flat spacetime and up to now it isn't answered because Marcus reactions to this spin-2 graviton idea is the above.

Well. So how do spin-2 gravitons (plus gravity fields) over flat spacetime explains Big Bang expansion? I guess we can consider the spin-2 fields as unique in that the fields can expand. Remember the Inflaton is also a field.. so it fills the gravity fields with inflatons expanding the fields with the effect as like producing spacetime curvature (but not really). Isn't it?

About Loop Quantum Gravity. So we can also consider it as spin networks producing the right coupling of gravity and hence can also be consider as having graviton spin-2 field over flat spacetime. Meaning spacetime only appears curved in LQG but not really curved?? This was what I was asking Marcus prior to his reply above whether one can consider LQG as like String theory where it is about spin-2 gravity fields over flat spacetime with the curvature geometry as not really a priori. But Marcus, like fellow poster surprised, misunderstood the concept as I didn't add the gravity field (behind spin 2) idea so didn't answer it. So let me ask this again now so someone can answer this LQG question above and we can close this thread clean. Thanks.